Clathrate hydrates are crystalline compounds that occur when water forms a cage-like structure around guest molecules, particularly gaseous molecules. Clathrate hydrates, especially in the petroleum industry, are referred to as gas hydrates, gas hydrate crystals, or simply hydrates. In the petroleum industry, gas hydrates pose particular problems with respect to producing, transporting, and processing of natural gas in petroleum fluids. Typical gas hydrates formed in petroleum (hydrocarbon) environments are composed of water and one or more guest molecules such as methane, ethane, propane, isobutane, normal butane, nitrogen, carbon dioxide, and hydrogen sulfate. It is also known that other guest molecules such as ethane, nitrous oxide, acetylene, vinyl chloride, ethyl bromide, oxygen, etc., can also form clathrate hydrates.
It is well established that gas hydrate crystals, when allowed to form and grow, can become a nuisance at best and pose a serious problem at worst. Gas hydrates can block transmission lines and plug blowout preventers, jeopardize the foundations of deep water platforms and pipelines, collapse tubing and casing, and foul process heat exchangers and expanders. To overcome these problems, several thermodynamic measures are possible in principal: removal of free water, maintaining an elevated temperature and/or reduced pressure, or the addition of freezing point depressants (antifreeze). As a practical matter, the last mentioned measure, i.e., adding freezing point depressants, has been most frequently applied. Thus, lower alcohols and glycols, e.g., methanol, have been added to act as antifreezes. However, in order for such substances to be effective, it is necessary that they be added in substantial amounts, e.g., 30% by weight of the water present. Not only is this expensive, it poses an additional problem since the addition of such larger amounts of antifreeze requires that they be recovered prior to further processing of the fluid mixture, e.g., the gas-water mixture.
It has been known for some time that in lieu of antifreezes, one can employ a crystal growth inhibitor that inhibits the formation of the gas-hydrate crystals and/or the agglomeration of the gas hydrate crystallites to large crystalline masses sufficient to cause plugging. Thus, surface active agents such as phosphonates, phosphate esters, phosphonic acids, salts and esters of phosphonic acids, inorganic polyphosphates, salts and esters of inorganic polyphosphates, polyacrylamids, and polyacrylates have been used.
It is also known that poly-N-vinyl-2-pyrrolidone (PVP), which is a well-known water-soluble polymer, is effective, in relatively low concentrations, in interfering with the growth of gas-hydrate crystals. WO94/12761, published Jun. 9, 1994, discloses other additives for inhibiting crystal growth and controlling the formation of gas hydrates in fluid mixtures such as are encountered in the hydrocarbon industry in the production, transportation, and processing of petroleum and natural gas fluids.